1. Field of the Invention
The present invention relates to a flow rate measuring apparatus, and particularly relates to a flow rate measuring apparatus for measuring flow rate of intake air of an internal combustion engine.
2. Description of the Related Art
A known flow rate measuring apparatus that is mounted on an intake pipe of an internal combustion engine and measures flow rate of intake air is a flow rate detecting device placed in a bypass passage through which part of intake air is taken in. For the flow rate detecting device, a semiconductor device having a flow rate detecting part formed of a thin film with a thickness of several micrometers is used.
Generally, the intake pipe of the internal combustion engine has an air cleaner placed therein for air purification. However, relatively small dust, for example, with a particle diameter of 100 μm or less cannot be removed by a purifying filter in the air cleaner and gets mixed into intake air. Furthermore, deterioration of the purifying filter, use of the purifying filter in a region with a severe usage condition, use of a substandard filter other than a proper product, or the like may cause relatively large dust with a particle diameter of 100 to 200 μm or so to get mixed into intake air.
When such dust is accelerated by intake air and collides with the flow rate detecting device at high speed, the flow rate detecting part may be damaged depending on the particle diameter or speed of the dust. In view of the above, Patent Documents 1 to 7 present structures as follows of the flow rate measuring apparatus including a semiconductor-type flow rate detecting device in order to prevent the flow rate detecting part from being damaged by incoming dust mixed with intake air.
According to the Patent Documents 1 and 2, a bypass passage through which part of flow rate of intake air is taken in is bent several times, then the flow rate detecting device is placed in the bypass passage downstream of the bends. This structure prevents dust accelerated by the mainstream from directly colliding with the flow rate detecting device. On the other hand, dust entering the bypass passage is decelerated by colliding with the wall surface of the bypass passage several times, which reduces energy of collision with the flow rate detecting device.
According to the Patent Document 3, the wall surface of the bypass passage is formed into roughened surface including continuous bumps and dips with a pitch of 50 μm to 1 mm. This reduces rebound when dust entering the bypass passage collides with the wall surface to decelerate the dust, thereby reducing energy of collision with the flow rate detecting device.
According to the Patent Document 4, bumps and dips in sawtooth pattern are molded on the wall surface of the bypass passage in the range on which the inlet opening plane of the bypass passage is projected. Due to this, dust accelerated by intake air collides straight with the sawtooth wall surface several times to be decelerated, which reduces energy of collision with the flow rate detecting device.
According to the Patent Document 5, a partition plate for branching a stream is formed in the bypass passage curved in a U-shape. This causes fluid to be measured that is introduced in the bypass passage to largely turn around before reaching an inlet of a sub-bypass passage in which the flow rate detecting device is placed, which prevents largely inertial dust from entering the sub-bypass passage.
According to the Patent Document 6, a guide louver for stopping dust rebounding from the wall surface of the bypass passage is placed upstream of the flow rate detecting device to prevent dust from colliding with the flow rate detecting device placed inside the guide louver. This reduces damage of the flow rate detecting device and adherence of contaminant to the flow rate detecting device.
According to the Patent Document 7, a plate-like member having a plate surface parallel to the flow passage direction of the bypass passage is provided directly upstream of the flow rate detecting device. This plate-like member smoothes the fluid to be measured, and also causes dust included in the fluid to be measured to move with the smooth flow, so the dust will no longer have a large momentum in the direction perpendicular to the detection surface of the flow rate detecting device. This reduces damage of the flow rate detecting device and adherence of contaminant to the flow rate detecting device.    Patent Document 1: Japanese Patent No. 4,161,077    Patent Document 2: Japanese Patent No. 3,602,762    Patent Document 3: Japanese Patent No. 4,553,898    Patent Document 4: Japanese Patent No. 4,929,335    Patent Document 5: Japanese Patent No. 3,681,627    Patent Document 6: Japanese Patent No. 3,797,210    Patent Document 7: JP-A-2003-315116
As described above, for the conventional flow rate measuring apparatus, various structures have been proposed in order to prevent the flow rate detecting device from being damaged by dust mixed into air as fluid to be measured. On the other hand, since the trajectory of flying dust varies depending on the particle diameter of the dust, a structure having high tolerance for dust having various particle diameters is needed. Also, a structure that increases dust tolerance while not decreasing the flow rate detection accuracy is needed.
However, in the flow rate measuring apparatus of the Patent Documents 1 and 2, increasing the number of bends in the bypass passage increases pressure loss occurring in the bypass passage, thereby causing drift. This causes turbulence in air moving toward the flow rate detecting device, thereby decreasing the flow rate detection accuracy. Thus, only bending the bypass passage has difficulty in sufficiently reducing collision energy of dust while maintaining the flow rate detection accuracy.
Furthermore, the flow rate measuring apparatus of the Patent Document 3 has a problem that collision energy of relatively large dust with a particle diameter of 100 to 200 μm or so cannot be sufficiently reduced. Also, providing fine bumps and dips on the wall surface of the bypass passage makes it easier for dust of several micrometers to adhere to the wall surface, which causes a problem of increase in characteristic variation due to contamination and a problem of not obtaining an intended effect due to bumps and dips covered with contaminant.
Furthermore, the flow rate measuring apparatus of the Patent Document 4 also has a problem that collision energy of dust with a particle diameter of 100 to 200 μm or so cannot be sufficiently reduced, similarly to that of the Patent Document 3. Also, the sawtooth bumps and dips having an angle less than 90 degrees are likely to cause stagnation of air, which causes a problem of increase in pressure loss and a problem of decrease in the flow rate detection accuracy due to air turbulence. Another problem is that small dust is likely to be deposited on the bumps and dips, and, when the bumps and dips are covered with the dust, an intended effect cannot be obtained.
Furthermore, the flow rate measuring apparatus of the Patent Document 5 includes the flow rate detecting device placed in the deepest part of the U-shaped sub-bypass passage, which has a problem that, when air containing dust is introduced into the sub-bypass passage, the airborne dust flies along the sub-bypass passage and generally perpendicularly collides with the flow rate detecting device.
Furthermore, in the flow rate measuring apparatus of the Patent Document 6, dust entering inside of the guide louver collides with the guide louver and then moves toward the flow rate detecting device, so collision with the flow rate detecting device cannot be avoided. Another problem is that the guide louver mounted near the flow rate detecting device causes air turbulence, thereby significantly reducing the flow rate detection accuracy.
Furthermore, in the flow rate measuring apparatus of the Patent Document 7, the plate-like member is provided directly upstream of the flow rate detecting device, which provides an effect of smoothing the flow velocity vector against drift, but has a problem of increase in pressure loss and significant decrease in the speed of air flow in the bypass passage and a problem of large air turbulence directly downstream of the plate-like member causing decrease in the flow rate detection accuracy.